The OLED diode structure was discovered in 1987 and has been developed into a technology that is used for displays in cellphones, microdisplays and televisions. Microdisplays are small displays that are used under magnification or for projection applications. Some of the applications of microdisplays are camera viewfinders and wearable displays, specifically for professions that require hands free information or for virtual reality applications.
One feature that differentiates microdisplays from normal displays is that the pixel density of microdisplays is much greater than normal displays. An example of normal displays with high pixel densities are the displays on the Samsung Galaxy S4 and the HTC One with 441 and 468 pixels per inch (PPI), respectively. Pixel densities on some micro-displays such as eMagin™ SXGA and Holoeye™ HED 6001 have 2646 and 3175 PPI, respectively, about an order of magnitude higher in PPI than normal displays.
Most microdisplays are fabricated on single crystal silicon substrates using CMOS technology. For most microdisplays, liquid crystals (LC) with backlighting are used, but a growing segment of microdisplays are based on organic light emitting diodes (OLEDs). OLED microdisplays have a few advantages over LC microdisplays: OLED is intrinsically simpler in design, does not need backlighting, has a higher viewing angle, faster response time, higher contrast, and it has a higher temperature operating range.
A full color (i.e. RGB colors) OLED display requires patterning to place individual color OLED stacks onto its respective color sub-pixels. Conventional patterning using lithography is not possible with organics due to the sensitivity of the organics to photoresist and chemistries, which is required for photoresist processing. There are few methods to pattern organics other than photoresist such as metal shadow mask, organic vapor jet deposition, and thermal dye transfer. All commercial high resolution full color OLED displays are made using shadow mask patterning in which fine metal masks (FMM) are used.
Current methods produce color emitter layers with a metal shadow mask, which suffers from resolution issues and may prevent color emitter sub-pixel sizes under 10 microns. Other methods create high-resolution OLED micro-display RGB sub-pixels using a white OLED overlaid with a color filter. The color filters may absorb approximately 80% of the white light and reduce light output from the OLED.
FIG. 1 illustrates a schematic structure of typical OLED with patterned anode and un-patterned organic layers, cathode and thin film encapsulation with red, green and blue color filters. Such a conventional OLED structure includes one or more anodes disposed on a silicon backplane substrate, where deposited on the one or more anodes are: a hole injection layer, a hole transport layer, at least one organic light-emitting layer (e.g., blue-green EML and red-orange EML layers), an electron transport layer, a cathode, a thin film encapsulation layer, and at least one color filter (e.g., red, green, and blue CF layers) disposed on the thin film encapsulation layer. Top emitting active matrix organic light emitting diode (AMOLED) displays require an anode to inject charges (holes) into subsequently deposited organic layers in a structure as indicated in FIG. 1. This anode is typically composed of a highly reflective metal with an appropriate work function that matches with the energy levels of the hole injection layer for efficient charge injection. In an active matrix display, this anode must be patterned on top of the backplane, typically using lithography or an etch process to define the subpixel. For AMOLED microdisplays, these pixels can be <10 microns in size so a micron scale patterning technique is required. Defining the anode pixel array with existing shadow mask technology is not possible for small pixels (<10 micron) as the state-of-the-art fine metal masks are not flat enough or to accurately define <10-micron feature sizes.
The processing required to pattern these layers through photolithography or etches poses several problems. First, it introduces additional process steps and complexity, requiring multiple tools to deposit and pattern the metal. These process steps increase the tact time required per display and introduce additional cost for materials and equipment. Second, the processing required to pattern the deposited metal modifies its surface as an undesired by-product of the patterning process, changing its reflectivity, resistivity and work function and thus its performance as an anode in OLEDs. As these steps typically occur in air, surface oxides may also form. These issues preclude the use of many materials, require careful tailoring of the anode to withstand processing, and frequently require additional processing steps to treat the surface after patterning. Finally, these additional steps as well as the act of removing metal can frequently leave residual metal, particles or residues which may introduce defects and reduce yield or require additional steps to attempt to eliminate these defects. For all these reasons, existing anode patterning techniques are sufficient but non-satisfactory for defining the anode pixel array.